The invention relates to the field of ringing controllers, more specifically to ringing controllers that provide reduced noise switching between battery feed and ringing states.
With increased demand to deliver high-speed data services to subscribers, many techniques have been developed to use existing telephone loops to carry digital subscriber line (DSL) signals simultaneously with normal voice-band telephony services. Generally, the signals associated with plain old telephone service (POTS), such as on-hook and off-hook transitions and ringing, generate high noise levels at frequencies above the voice band and are thus incompatible with high-speed DSL signals. Traditionally, large and costly filters, often referred to as POTS splitters, have been employed at the telephone equipment location to remove POTS-created noise from the data band. This requirement has been an impediment to the large-scale deployment of DSL services.
The generally-accepted method of alerting a subscriber that an incoming telephone call has arrived is to apply a high magnitude alternating current (AC) waveform to the subscriber loop in order to ring a bell or similar audible alerting device at the subscriber's premises. One very common standard in North America is to use 86 V rms AC signals with a frequency of 20 Hz, although other different voltages and frequencies can be employed. This AC waveform is often referred to as the power-ringing signal.
The process of ringing a subscriber's line can be considered as a cycle between two states. First, the state of providing a battery feed to the loop to await activity, which may include on-hook transmission or supervision. Second, the state of providing the power-ringing signal to the loop. Digital Subscriber Loop (DSL) signals require very good signal-to-noise ratios (SNRs) to achieve the high throughputs required by the applicable DSL standards and hence are quite susceptible to noise, both in the time and frequency domains. Hence, it is desirable that transitions between the battery feed and ringing states cause a minimum amount of noise.
Traditionally, mechanical relays have been employed to switch ringing signals onto the subscriber loop. It is an unfortunate characteristic of relays that they tend to introduce discontinuities onto the loop voltage due to timing variations, abrupt switching behavior, contact bounce, or open-circuit intervals between states. Large voltage discontinuities manifest themselves as high frequency noise, which interfere with DSL signals.
More recently, solid state relays have been employed to switch ringing signals but, as yet, their timing and transition characteristics have tended to induce noise levels incompatible with DSL.
It is desirable that the change between the battery feed state and the ringing state be continuous and smooth to avoid creating noise artifacts that interfere with DSL transmission. A need therefore exists for a ringing controller that will allow for reduced noise switching between battery feed and ringing states. Consequently, it is an object of the present invention to obviate or mitigate at least some of the above-mentioned disadvantages.